This invention is related in general to the field of electrical and electronic circuits. More particularly, the invention is related to apparatus and a method for quadrature tuner error correction.
A block diagram of an ideal quadrature tuner 10 and digitizer 12 is shown in FIG. 1. An unmodulated input carrier signal 14 is represented by A cos (xcfx89Ct+xcex8). Unmodulated input carrier signal 14 has a peak amplitude A, radian frequency xcfx89C, and an arbitrary phase of xcex8 radians. Input carrier signal 14 is mixed by mixers 16 and 17 with two signals 18 and 19 generated by a local oscillator (LO) 20. Signals 18 and 19 are represented by cos(xcfx89LOt) and xe2x88x92sin(xcfx89LOt), which both have a normalized amplitude of 1, radian frequency xcfx89LO, and normalized phase of 0 radians. The resulting signals are:
A cos(xcfx89Ct+xcex8)cos(xcfx89LOt)=(A/2)cos[(xcfx89Cxe2x88x92xcfx89LO)t+xcex8]+(A/2)cos[(xcfx89C+xcfx89LO)t+xcex8]
and
xe2x88x92A cos(xcfx89Ct+xcex8)sin(xcfx89LOt)=(A/2)sin[(xcfx89Cxe2x88x92xcfx89LO)t+xcex8]xe2x88x92(A/2)sin[(xcfx89C+xcfx89LO)t+xcex8]xe2x80x83xe2x80x83(1)
Ideal lowpass filters (LPFs) 24 and 25 receive these resultant signals and remove only the high frequency components, resulting in analog in-phase (I) and quadrature (Q) signals 26 and 27:
I=(A/2)cos[(xcfx89Cxe2x88x92xcfx89LO)t+xcex8]
and
Q=(A/2)sin[(xcfx89Cxe2x88x92xcfx89LO)t+xcex8]xe2x80x83xe2x80x83(2)
Analog in-phase and quadrature signals 26 and 27 are converted to digital form by analog-to-digital converters (ADC) 28 and 29 for further processing.
On the other hand, a real quadrature tuner produces offset, gain, and phase errors which reduce the accuracy of the resulting in-phase and quadrature signals. The worst case gain and phase errors of several commercially available tuner devices are in the xc2x10.5 dB and xc2x15.0xc2x0 range, respectively.
Conventional attempts to correct these errors may use. analog trimming, adjustments, or calibration. These methods are undesirable due to high cost. Another conventional method uses analog-to-digital converters with wide bandwidths and high speed to digitize the in-phase and quadrature output signals. The requirement of high performance analog-to-digital converters adds substantially to cost and the complexity of the circuit.
Accordingly, there is a need for a circuit and method that efficiently correct quadrature tuner offset, gain, and phase errors. In accordance with the present invention, apparatus and a method of quadrature tuner error correction are provided which eliminate or substantially reduce the disadvantages associated with prior circuits or algorithms.
In one aspect of the invention, apparatus for quadrature tuner error correction includes an offset correction portion adapted to receive a digital in-phase signal and a digital quadrature signal from a quadrature tuner. The offset correction portion has an in-phase portion comprising a summer adapted to receive the digital in-phase signal, subtract an in-phase offset estimate therefrom, and generate an offset corrected in-phase signal, and a feedback loop adapted to integrate the offset corrected in-phase signal, multiply the integrated offset corrected in-phase signal by a first adjustable constant, and generate the in-phase offset estimate. The offset correction portion has a quadrature portion comprising a summer adapted to receive the digital quadrature signal, substract a quadrature offset estimate therefrom, and generate an offset corrected quadrature signal, and a feedback loop adapted to integrate the offset corrected quadrature signal, multiply the integrated offset corrected quadrature signal by a second adjustable constant, and generate the quadrature offset estimate.
In another aspect of the invention, a method of correcting errors in a quadrature tuner includes an offset correction process which receives a digital in-phase signal and a digital quadrature signal from the quadrature tuner. The offset correction process has an in-phase portion which includes the steps of receiving the digital in-phase signal, subtracting an in-phase offset estimate therefrom, and generating an offset corrected in-phase signal, and feeding back the offset corrected in-phase signal through a first feedback loop, the feedback loop integrating the offset corrected in-phase signal, multiplying the integrated offset corrected in-phase signal by a first adjustable constant, and generating the in-phase offset estimate. The offset correction process also has a quadrature portion with the steps of receiving the digital quadrature signal, subtracting a quadrature offset estimate therefrom, and generating an offset corrected quadrature signal, and feeding back the offset corrected quadrature signal through a feedback loop, the feedback loop integrating the offset corrected quadrature signal, multiplying the integrated offset corrected quadrature signal by a second adjustable constant, and generating the quadrature offset estimate.
In yet another aspect of the invention, apparatus for quadrature tuner error correction includes an offset correction portion adapted to receive a digital in-phase signal and a digital quadrature signal from a quadrature tuner and generating offset corrected in-phase and quadrature signals, a gain correction portion coupled to the offset correction portion adapted to receive the offset corrected in-phase and quadrature signals and generating offset and gain corrected in-phase and quadrature signals, and a phase correction portion coupled to the gain correction portion adapted to receive the gain corrected in-phase and quadrature signals and generating offset, gain and phase corrected in-phase and quadrature signals.